1. Field of the Invention
The present invention relates to a coating method for various media for recording images or data, such as magnetic tape, photographic film, photographic paper, etc., and particularly relates to a coating method based on a rod-coat system.
2. Description of the Related Art
Conventionally, known rod-coat systems or known bar-coat systems are roughly classified into three, that is, one that which uses a rod having a smooth and cylindrical outer-circumferential surface, one that which uses a rod wound with a wire, and one that which uses a rod provided with a narrow groove, as schematically explained, for example, Yuji Harazaki, xe2x80x9cCoating Systemsxe2x80x9d, Chapter xe2x80x9cRod Coaterxe2x80x9d, Maki Shoten Co., Oct. 30, 1979, pp 51-56.
The coating method is to measure and smooth coating liquid excessively applied in advance to one surface of a flexible long-sized band-shaped support (hereinafter referred to as xe2x80x98webxe2x80x99) transferred continuously, and to provide a coating layer.
Such a rod wound with a wire or a rod provided with a narrow groove is suitable for a method in which a coating liquid excessively applied in advance to one surface of a flexible long-sized band-shaped support (hereinafter abbreviated to xe2x80x9cwebxe2x80x9d) transferred continuously is measured in accordance with the diameter of the wire or the depth of the groove and then smoothed so that a comparatively large amount of coating film is provided. However, wale streaks often appear in the coated surface easily, and there is a problem in durability and manufacturing cost of the rod itself.
The rod having a smooth and cylindrical outer-circumferential surface, which can be manufactured at a low cost and has a long life, is suitable for a method in which a comparatively small amount of coating film is provided. A coating apparatus using such a rod usually had such a configuration as shown in FIG. 4.
That is, according to a conventional coating apparatus RC, a web W is transferred continuously in the direction of the arrow C along a web track formed by a plurality of web guide rollers GR, while a precoating layer CLa with an excessive amount is applied to one surface of the web W laid between the web guide rollers GR by a precoating apparatus constituted by an applicator roll AR partially immersed in coating liquid Liq in a coating pan P and driven to rotate in a direction CC reverse to the transferring direction of the web W. After that, a rod portion Rod held in a recess portion at the upper end of a fixed stand ST and having a smooth and cylindrical outer-circumferential surface or a rod portion Rod wound with a wire is pressed against the precoating layer CLa to thereby scrape surplus coating liquid CLb off, so that a final coating layer CLc measured to a desired amount of coating is provided.
In many cases, the surplus coating liquid CLb scraped off by the rod portion Rod was not drained directly, but it was recovered in a liquid receiving tank RT, mixed with the coating liquid Liq supplied from a coating liquid supply source (not-shown), delivered to the coating pan P through a plunger pump PP, and then reused in order to reduce the cost.
A coating amount h of the final coating layer CLc in the case where the rod portion Rod had a smooth and cylindrical outer-circumferential surface was obtained on the basis of Foil""s expression, that is, hxe2x88x9dR(Vxc2x7xcex7/T)⅔. In this expression, h designates the coating amount (xcexcm) of the final coating layer CLc; R, the diameter (mm) of the rod portion; V, the web transferring speed (m/min); xcex7, the viscosity (cp) of the coating liquid; and T, the web tension (kgf/m).
In addition, in the case of a rod wound with a wire or a rod provided with a narrow groove h designates the thickness of the rod in accordance with the Foil""s expression under the condition that the groove portion is removed. This is defined as a virtual coating amount.
However, in the coating method based on the above-mentioned background-art rod-coat system, because of the factors such as the web tension, the diameter of the rod portion Rod, the viscosity of the coating liquid Liq, the transferring speed of the web W, the coating amount of the precoating layer CLa, the coating amount of the final coating layer CLc, and so on, scraping mist occurs in the area where the precoating layer CLa has been scraped off on the upstream side of the rod portion Rod. Consequently, coating unevenness like herringbone-twill is induced in the surface of the final coating layer CLc. Particularly, when the coating liquid Liq containing carbon particles and/or abrasive, etc. is used in a back layer for giving properties of shading or properties of transferring, the point that light is transmitted easily and transferring of the web is made rough due to the above-mentioned coating unevenness has not been solved completely.
In addition, when the precoating apparatus is constituted by an applicator roll, the maximum speed is limited to 180 m/min, so that it is impossible to perform coating at a higher speed. Accordingly, there has been a limit to improvement of productivity. Further, in the applicator roll, the amount of precoating is determined by the diameter of the roll so as not to be controlled desirably. Accordingly, in order to manufacture many sorts of products without producing coating unevenness like herringbone-twill, it has been necessary to exchange the applicator roll in accordance with the sort. Further, when the rod portion is made of a flat bar, ribs appear in the coating and the distribution of thickness in the width direction also deteriorates.
Further, in the above-mentioned coating apparatus based on the above-mentioned background-art rod-coat system, abrasion wear of the rod portion per unit time increased sharply as the web transferring speed was made higher in order to improve the productivity and further as the hardness of abrasive added to a magnetic recording layer increased. As a result, unevenness of the thickness of the final coating layer or a rib fault in the surface of the final coating layer often appeared, while the life of the rod portion was reduced. Accordingly, not only the time when a coating line suspended increased for exchanging the rod portion, but also the manufacturing cost was apt to increase.
Particularly, the abrasion resistance of a rod wound with a wire or a rod provided with a narrow groove was low. This was because only a wire of comparatively soft material such as SUS can be wound on a rod.
In addition, when the rod portion was made to be a flat bar, ribs appeared in the coating layer.
It is an object of the present invention to provide a coating method based on a rod-coat system which is superior in the quality of a coating surface, capable of high speed coating, and convenient to exchange the sort of products.
It is another object of the present invention to provide a coating apparatus based on a rod-coat system which can improve the abrasion resistance of a rod and which can ensure superior surface quality without producing any ribs.
According to a first aspect of the present invention, in a coating method, one surface of a flexible long-sized band-shaped support laid between a plurality of web guide rollers forming a portion of a web track is coated with a precoating layer with an excessive amount by a precoating apparatus while the flexible band-shaped support is transferred continuously along the web track, and thereafter a rod portion is pressed against the precoating layer to thereby scrape surplus coating liquid off so as to lay a final coating layer measured to a desired amount, wherein a leading end surface of an extrusion-type head portion is pressed against the one surface of the flexible band-shaped support laid between the web guide rollers so that the precoating layer is applied thereto.
According to a second aspect of the present invention, in the coating method of the first aspect, a relation between a coating amount of the precoating layer and a coating amount of the final coating layer satisfies an expression [1]:
6xcex7xe2x88x920.3xe2x89xa6H/hxe2x80x83xe2x80x83[1]
where H designates the coating amount of the precoating layer (cc/m2); h, the coating amount and virtual coating amount of the final coating layer; and xcex7, viscosity (cp) of the coating liquid.
According to a third aspect of the present invention, in the coating method of the first or second aspect, a flat bar having a rod diameter not larger than 6 mm is used as the rod portion to scrape the surplus coating liquid off.
According to a fourth aspect of the present invention, in the coating method of any one of the first to third aspects, the coating amount of the final coating layer is made to be not larger than 15 cc/M2.
According to a fifth aspect of the present invention, in a coating apparatus, one surface of a long-sized web laid between a plurality of web guide rollers forming a portion of a web track is coated with a precoating layer with an excessive amount by a precoating apparatus while the web is transferred continuously along the web track, and thereafter a rod portion is pressed against the precoating layer to thereby scrape surplus coating liquid off so as to lay a final coating layer measured to a desired amount; in that the rod portion has an outer-circumferential surface with Vickers hardness (Hv) of 1,000 or more.
According to a sixth aspect of the present invention, in a coating apparatus of the fifth aspect, the rod portion is a flat rod.
According to a seventh aspect of the present invention, in a coating apparatus of the fifth or sixth aspect, the rod portion comprising a super hard alloy.
According to an eighth aspect of the present invention, in a coating apparatus of any one of the fifth to seventh aspects, the rod diameter of the rod portion is not larger than 20 mm.
According to the above-mentioned coating method of the first aspect, entrained air increasing as the transferring speed of the web becomes higher is prevented from entering a coating point near the leading end surface of the extrusion-type head portion by the meniscus of the coating liquid formed in a slight gap between the leading end surface of the extrusion-type head portion and the concave one surface of the web. As a result, the precoating layer having an excessive amount is applied stably without turbulence, at the coating point, due to the entrained air.
The rod portion in the present invention may be any one of a flat rod having a smooth and cylindrical outer-circumferential surface, a wire rod wound with a wire, and a rod provided with a groove in its cylindrical outer-circumferential surface.
Further, according to the above-mentioned coating method of the second aspect, the relation between the coating amount of the precoating layer and the coating amount of the final coating layer satisfies the expression [1]. Accordingly, scraping mist is restrained from being produced in the area where the precoating layer is scraped off on the upstream side of the rod portion. As a result, coating unevenness like herringbone-twill is prevented from being produced on the surface of the final coating layer.
6xcex7xe2x88x920.3xe2x89xa6H/hxe2x80x83xe2x80x83[1]
In the expression [1], H designates the coating amount of the precoating layer (cc/m2); h, the coating amount and the virtual coating amount of the final coating layer (cc/m2); and xcex7, the viscosity (cp) of the coating liquid.
The inventors of the present invention carried out various coating experiments while changing the viscosity of the coating liquid, the coating amount of the precoating layer, and the coating amount and the virtual coating amount of the final coating layer. As a result, the inventor found out that coating superior in surface quality could be performed if the above-mentioned values satisfied the relation of the expression [1].
In addition, since the precoating apparatus is formed as an extrusion-type head at this occasion, it is possible to ensure a wide region where the precoating layer can be applied, so that various sorts of products can be manufactured without making a change in equipments and without producing coating unevenness like herringbone-twill.
Further, according to the above-mentioned coating method of the third aspect, a flat bar the rod diameter of which is not larger than 6 mm is used as the rod portion so as to scrape the surplus coating liquid off. It is therefore possible to avoid ribs.
Further, according to the above-mentioned coating method of the fourth aspect, the coating amount of the final coating layer is made not larger than 15 cc/m2. It is therefore possible to form a coating layer which is uniform in the width direction.
According to the above-mentioned coating apparatus of the fifth aspect, the rod portion has an outer-circumferential surface with Vickers hardness (Hv) of 1,000 or more. Accordingly, the abrasion wear of the rod portion per unit time is greatly restrained from increasing, so that the life of the rod portion can be made much longer.
Further, according to the above-mentioned coating apparatus of the sixth aspect, the rod portion is a flat rod, so that the abrasion resistance of the rod portion is more improved.
Further, according to the above-mentioned coating apparatus of the seventh aspect, the rod portion is formed of a super hard alloy, so that the abrasion resistance of the rod portion is more improved.
Further, according to the above-mentioned coating apparatus of the eighth aspect, the diameter of the rod portion is not larger than 20 mm, so that it is possible to restrain ribs from appearing.